One of the most common procedures for a dentist is the repair of a broken tooth. When a tooth is broken, a portion of the enamel comes off, exposing the dentin underneath. The dentin must be covered to prevent the dentin from becoming infected. The dentist will grind a portion of the remaining enamel away to prepare the tooth for a crown. Once the grinding procedure is complete, a reduced stump remains and a mold of the stump is made with a quick setting mold material. Further a mold of the adjacent teeth and the opposing teeth are also made. Then a temporary crown in placed on the stump. The temporary crown has been partially customized to fit over the stump and to mesh with the opposing teeth. However, due to traditional time constraints, the temporary crown rarely feels as natural as the original tooth. Further, the temporary crown must be affixed to the stump with a temporary fixative.
With the mold as a guide, an outside laboratory will prepare a permanent crown. The permanent crown may be made of porcelain, gold, ceramic, or other metal or substance. This process usually takes at least three weeks to complete. During this time, the patient must function with the temporary crown. Unfortunately, there is a risk that the temporary crown may loosen and be swallowed or otherwise lost by the patient. Even if it only loosens, bacteria can gain access to the dentin for a time and cause more serious dental health issues. Also, once the permanent crown is available for placement, the temporary crown must be removed. This requires the dentist to twist the temporary crown off the stump, exerting a significant torque to the roots. Even then, if the permanent crown is misshaped, then it may need to be removed again and remade.
A need exists for a method of improving the speed of producing a permanent crown for a patient. Indeed, if the crown could be produced while the patient waited, it would be a great savings for the patient and the dentist both. Moreover, it would also be beneficial to eliminate the need to make a temporary crown at all. Sirona Dental Services GmbH, of Bensheim Germany produces a milling machine specifically for producing porcelain crowns. It is disclosed in U.S. Pat. No. 6,394,880 discloses one aspect of this milling machine. It allows for the use of two milling bits to simultaneously work a blank to form it into a permanent crown. The mill bits are located on opposite sides of the blank and can move in an x-, y-, and z-plane. However, the bits can not be changed on demand to accommodate a different blank material. Also, there is no method disclosed for determining the bit wear to warn the dentist that the crown's dimensions may be skewed due to bit wear.
The Sirona patent illustrates a portion of a larger milling machine known as the CEREC. The CEREC has several other drawbacks. First, it has only a wireless connection with an intraoral digitizer used to make the measurements of the stump and adjacent teeth. Once the measurements are made, the intraoral digitizer cannot be used until the crown is finished. Therefore, a need exists for a milling machine that includes a memory that can store the required data thereby freeing the intraoral digitizer to be used again. Further, the CEREC device is flawed in its failure to minimize vibration that affects the quality of the milling. Even minor vibration can create many microns of error on the surface of the crown.